The present invention is based on a power supply having two switched-mode power supplies. The first switched-mode power supply comprises in particular a current pump with a coil for an active power factor correction and the second switched-mode power supply comprises a power factor coil in its current supply.
It is known for entertainment electronic appliances to use two switched-mode power supplies to supply power to the very wide range of circuit components, which switched-mode power supplies are connected in parallel in order to provide the necessary number of supply voltages, and/or in order to make it possible to switch off specific circuit groups as a function of a chosen operating mode. In television sets, for example, which have an Internet module or a digital decoder, it is worthwhile using a separate switched-mode power supply for this circuit unit, which separate switched-mode power supply is switched on and off as required, with this unit.
Switched-mode power supplies result in a highly pulsed load on the line network, which leads to harmonic currents in the line network. This load occurs in the region of the voltage maximum of the sinusoidal mains voltage, in which region an energy storage capacitor in the switched-mode power supply is recharged. Appliances having a relatively high power consumption, such as relatively large television sets, must therefore now satisfy specific regulations with regard to the harmonic currents. The harmonic load on the line network from an appliance can in this case be specified by a so-called power factor.
It is known for a so-called “preconverter” to be used for active power factor correction in order to reduce the harmonic load in the line network, which preconverter is connected upstream of the switched-mode power supply and has a coil to which a current is applied cyclically by a switch. This results in a quasi-continuous current flow from the line network.
Furthermore, switched-mode power supplies are known which use power factor correction with a so-called current pump, for example from DE-A-196 10 762, EP-A-0 700 145 and U.S. Pat. No. 5,986,898. These have a first current path on the primary side, through which an energy storage capacitor in the switched-mode power supply is charged via a diode and a mains rectifier in the switched-mode power supply, and have a second current path with an inductance which is arranged between the mains rectifier and the primary winding of the transformer. The energy storage capacitor is in this case charged predominantly via the second current path. The inductance of the second current path thus acts like a current pump which, controlled by the switching transistor, draws a continuous current or at least a broadened current flow from the line network, in order to improve the power factor. In this sense, these appliances are thus also switched-mode power supplies with active power factor correction since, in the case of these switched-mode power supplies as well, an at least broadened current flow is produced from the switched-mode power supply, with an improved power factor, via a coil and by means of a transistor, in this case the switching transistor of the switched-mode power supply.
A further switched-mode power supply with active power factor correction is known from EP-A-0 588 168, EP-A-0 588 172 and EP-A-0 588 173. This likewise has two current paths, with the first current path connecting the bridge rectifier via a diode directly to the primary winding of a transformer. The primary winding thus operates directly with the rectified mains voltage. An energy storage capacitor is arranged in the second current path and is discharged via a second switching transistor as a function of the first switching transistor, in order to improve the ripple on the output voltages from the switched-mode power supply.
It is also known that it is possible to improve the power factor by means of a coil which is arranged at the input of the switched-mode power supply, in particular upstream of the energy storage capacitor. This coil is also known as a mains frequency coil, 50 Hz coil or power factor coil. In order to avoid confusion with other coils, the term power factor coil is used throughout the description here.
However, this power factor coil requires a relatively large amount of inductance to achieve sufficiently good power factor correction. A further disadvantage is that, when the mains switch is operated in order to switch the appliance off, the current flow in the power factor coil is interrupted suddenly. The energy stored in the power factor coil must, however, be dissipated. Since the open mains switch represents the highest impedance in the circuit, a very high voltage is therefore developed across the switching contacts of the mains switch, leading to arcing. Depending on the configuration of the switch, the rate at which the switch opens may also be comparatively slow, so that the arc is produced until the end of that mains half-cycle. In this case, no high voltage is formed across the contacts, but, in both cases this means that the mains switch ages more quickly, and the switch represents a safety risk since, in the worst case, it becomes a potential fire source.
If an appliance has two switched-mode power supplies, but only one mains cable which connects the two switched-mode power supplies to the line network, then the appliance is regarded as a unit which must comply with the regulations for power factor correction.